带有中间件的示例机架应用程序

这是工作代码，您可以将其保存到名为app.rb的文件中，并在命令行中使用ruby app.rb在本地运行：

require 'rack'
require 'rack/server'

class EnsureJsonResponse
  def initialize(app)
    @app = app
  end

  # Set the 'Accept' header to 'application/json' no matter what.
  # Hopefully the next middleware respects the accept header :)
  def call(env)
    puts "JSON"
    env['HTTP_ACCEPT'] = 'application/json'
    @app.call env
  end
end

class Timer
  def initialize(app)
    @app = app
  end

  def call(env)
    puts "Timer"
    before = Time.now
    status, headers, body = @app.call env

    headers['X-Timing'] = (Time.now - before).to_s

    [status, headers, body]
  end
end

class HelloWorldApp
  def self.call(env)
    puts "HelloWorld"
    [200, {}, ["hello world"]]
  end
end

app = Rack::Builder.new do 
  use Timer # put the timer at the top so it captures everything below it
  use EnsureJsonResponse
  run HelloWorldApp
end

Rack::Server.start :app => app

输出

如果您向网页发出单一请求，您会看到以下内容：

+>ruby app.rb
>> Thin web server (v1.4.1 codename Chromeo)
>> Maximum connections set to 1024
>> Listening on 0.0.0.0:8080, CTRL+C to stop
Timer
JSON
HelloWorld
Timer
JSON
HelloWorld

为什么每个中间件以及底层应用都会在一次请求中调用两次？

Answer 1

大多数浏览器都倾向于请求图标。也许您想要更改我们的应用程序以找出所请求的路径：

class HelloWorldApp
  def self.call(env)
    puts "HelloWorld"
    puts "requested path: #{env["PATH_INFO"]}"
    [200, {}, ["hello world"]]
  end
end

Answer 2

我复制了您的应用并在本地运行。发生了这种情况：

[2014-03-08 22:01:29] INFO  WEBrick 1.3.1
[2014-03-08 22:01:29] INFO  ruby 2.0.0 (2013-06-27) [x86_64-darwin13.0.0]
[2014-03-08 22:01:29] INFO  WEBrick::HTTPServer#start: pid=74593 port=8080
Timer
JSON
HelloWorld
localhost - - [08/Mar/2014:22:01:33 PST] "GET / HTTP/1.1" 200 11
- -> /
Timer
JSON
HelloWorld
localhost - - [08/Mar/2014:22:01:33 PST] "GET /favicon.ico HTTP/1.1" 200 11
- -> /favicon.ico
Timer
JSON
HelloWorld
localhost - - [08/Mar/2014:22:01:33 PST] "GET /favicon.ico HTTP/1.1" 200 11

在我看来，您看到浏览器默默地请求favicon.ico文件。这可能吗？

FWIW，每当我在这个低级别调试某些东西时，我总会发现curl是一个有用的命令。

$ curl -i localhost:8080
HTTP/1.1 200 OK
X-Timing: 2.7e-05
Server: WEBrick/1.3.1 (Ruby/2.0.0/2013-06-27)
Date: Sun, 09 Mar 2014 06:03:58 GMT
Content-Length: 11
Connection: Keep-Alive

hello world%

导致服务器打印以下内容：

[2014-03-08 22:03:49] INFO  WEBrick 1.3.1
[2014-03-08 22:03:49] INFO  ruby 2.0.0 (2013-06-27) [x86_64-darwin13.0.0]
[2014-03-08 22:03:49] INFO  WEBrick::HTTPServer#start: pid=74662 port=8080
Timer
JSON
HelloWorld
localhost - - [08/Mar/2014:22:03:58 PST] "GET / HTTP/1.1" 200 11
- -> /

好哇！

Rack：为什么每个中间件在一次请求中被调用两次？

带有中间件的示例机架应用程序

输出

2 个答案: